The present invention generally relates to programmable metallization structures, and more particularly, to a programmable sub-surface aggregating metallization (xe2x80x9cPSAMxe2x80x9d) structure including an ion conductor, a plurality of electrodes and a voltage-controlled metal structure or dendrite formed through the ion conductor between the electrodes.
Memory devices are used in electronic systems and computers to store information in the form of binary data. These memory devices may be characterized into various types, each type having associated with it various advantages and disadvantages.
For example, random access memory (xe2x80x9cRAMxe2x80x9d) which may be found in personal computers is volatile semiconductor memory; in other words, the stored data is lost if the power source is disconnected or removed. Dynamic RAM (xe2x80x9cDRAMxe2x80x9d) is particularly volatile in that it must be xe2x80x9crefreshedxe2x80x9d (i.e., recharged) every few microseconds in order to maintain the stored data. Static RAM (xe2x80x9cSRAMxe2x80x9d) will hold the data after one writing so long as the power source is maintained; once the power source is disconnected, however, the data is lost. Thus, in these volatile memory configurations, information is only retained so long as the power to the system is not turned off.
CD-ROM is an example of non-volatile memory. CD-ROM is large enough to contain lengthy audio and video segments; however, information can only be read from and not written to this memory. Thus, once a CD-ROM is programmed during manufacture, it cannot be reprogrammed with new information.
Other storage devices such as magnetic storage devices (i.e., floppy disks, hard disks and magnetic tape) as well as other systems, such as optical disks, are non-volatile, have extremely high capacity, and can be rewritten many times. Unfortunately, these memory devices are physically large, are shock/vibration-sensitive, require expensive mechanical drives, and may consume relatively large amounts of power. These negative aspects make these memory devices non-ideal for low power portable applications such as lap-top and palm-top computers and personal digital assistants (xe2x80x9cPDAsxe2x80x9d).
Due to the rapidly growing numbers of compact, low-power portable computer systems in which stored information changes regularly, read/write semiconductor memories have become widespread. Furthermore, because these portable systems require data storage when the power is turned off, a non-volatile storage device is required. The simplest programmable semiconductor non-volatile memory devices in these computers are programmable read-only memory (xe2x80x9cPROMxe2x80x9d). The most basic PROM uses an array of fusible links; once programmed, a PROM cannot be reprogrammed. This is an example of a write-once read-many (xe2x80x9cWORMxe2x80x9d) memory. The erasable PROM (xe2x80x9cEPROMxe2x80x9d) is alterable, but each rewrite must be preceded by an erase step involving exposure to ultra violet light. The electrically erasable PROM (xe2x80x9cEEPROMxe2x80x9d or xe2x80x9cE2PROMxe2x80x9d) is perhaps the most ideal of conventional non-volatile semiconductor memory, as it can be written to many times. Flash memories, another type of EEPROM, have higher capacity than the low density, traditional EEPROMs but lack their endurance. One major problem with EEPROMs is that they are inherently complex. The floating gate storage elements that are used in these memory devices are difficult to manufacture and consume a relatively large amount of semiconductor real estate. Furthermore, the circuit design must withstand the high voltages necessary to program the device. Consequently, an EEPROM""s cost per bit of memory capacity is extremely high compared with other means of data storage. Another disadvantage of EEPROMs is that although they can retain data without having the power source connected, they require relatively large amounts of power to program. This power drain can be considerable in a compact portable system powered by a battery.
Accordingly, in view of the various problems associated with conventional data storage devices described above, it is highly desirable to have a read/write memory technology and device which is inherently simple and inexpensive to produce. Furthermore, this memory technology should meet the requirements of the new generation of portable computer devices by operating from a low voltage while providing high storage density, non-volatility, and a low manufacturing cost.
Electronic circuits may include literally millions of component parts. These component parts generally fall into two distinct categories, namely, passive components and active components. Passive components, such as resistors and capacitors, have electrical values associated with them which are relatively constant. On the other hand, some electrical characteristics of active components, such as transistors, are designed to change in response to an applied voltage or current.
Because of the extensive use of these two types of components, it is highly desirable to have a low-cost device which may perform both the functions of a passive component and an active component. For example, it would be highly desirable to have a device that acts as an active component which responds to an applied signal by altering its resistance and capacitance and yet, in an alternate embodiment, acts as a passive component which can be pre-programmed (i.e., the change is xe2x80x9crememberedxe2x80x9d by the device after programming is complete). Such a device would be used in many diverse applications from tuned circuits in communications equipment to volume controls in audio systems.
Because of the widespread use of devices such as memory devices, and programmable resistor and capacitor devices, it is very desirable to have a low cost, easy to manufacture device that may be implemented in all of these various applications, among others.
In accordance with an exemplary embodiment of the present invention, a programmable sub-surface aggregating metallization (xe2x80x9cPSAMxe2x80x9d) structure includes an ion conductor such as a chalcogenide-glass which includes metal ions and at least two electrodes (e.g., an anode and cathode) each having an electrically conducting material and disposed at opposing surfaces of the ion conductor. Chalcogenide materials as referred to herein include any compound including sulfur, selenium and/or tellurium. In an exemplary embodiment, the ion conductor is a composition formed from a chalcogenide and at least one Group I or Group II metal (most preferably, arsenic trisulphide-silver). The anode and cathode are each formed from any suitable conducting material, and the anode preferably contains some silver.
When a voltage is applied between the anode and cathode, a metal dendrite grows from the cathode through the ion conductor towards the anode. The growth rate of the dendrite may be stopped by removing the voltage or the dendrite may be retracted back towards the cathode by reversing the voltage polarity at the anode and cathode. When a voltage is applied for a sufficient length of time, a continuous metal dendrite grows through the ion conductor and connects the electrodes, thereby shorting the device. The continuous metal dendrite then can be broken by applying another voltage. The break in the metal dendrite can be reclosed by applying yet another voltage. Changes in the length of the dendrite or the presence of a break in the dendrite affect the resistance, capacitance, and impedance of the PSAM.